This invention relates to an apparatus for controlling a semiconductor laser used as a light source of various devices such as CD (Compact Disk) or LD (Laser Disk), etc.
An optical pick-up in which a semiconductor laser is used emits, onto a disc, laser beams respectively having intensities in conformity with those at the times of recording and reproduction to record information on the disc, and to reproduce the recorded information therefrom. At the time of the operation for recording information, a laser beam having a large light intensity is emitted from the optical pick-up onto the disc to record information on the disc. On the other hand, at the time of the operation for reproducing information, a laser beam having a light intensity smaller than that of a laser beam at the time of recording is emitted from the optical pick-up onto the disc to reproduce the recorded information on the basis of a signal light obtained so that the emitted laser beam is reflected on the disc surface. At the time of reproduction, by emitting, onto the disc, a laser beam at a maximum light intensity at which no information is recorded onto the disc, respective S/N ratios in a control signal of the servo system and a RF signal of the reproducing system is maintained at a sufficiently large value.
However, since the semiconductor laser is constructed as above to carry out the recording and reproducing operations, an oscillation spectrum might undesirably shift due to a difference between a light output at the time of recording and that at the time of reproduction.
In the case where the above-mentioned semiconductor laser is used as an optical pick-up, there may arise problems as described below due to the shift of the oscillation spectrum. First, when the optical system of the optical pick-up is constituted with a single spheric lens such as an aspherical lens, the focal position varies due to the shift of the oscillation spectrum. Secondly, when a prism for correcting the ellipsoidal emitting face of an outgoing beam of a laser beam emitted from the semiconductor laser is provided as an optical system, the laser beam varies in a certain direction by the shift of the oscillation spectrum. Thirdly, a laser beam is mode-hopped by the shift of the oscillation spectrum to generate noises. Fourthly, a laser beam intensity at the time of recording depends on a waveform in recording discs such as recordable CD (R-CD), etc. which uses coloring matter, so the laser beam undergoes the influence of the shift of the oscillation spectrum.
It is to be noted that in the case where an optical system in which the dispersion is small and the refractive index does not change even if the waveform shifts to any degree, such as an achromatic lens, etc. is used, there may take place a new problem such that the apparatus itself becomes expensive and large sized.